Organic solar cells are solar cell devices that use organic compounds in the active layer and the charge-transporting substances. The dye-sensitized solar cells developed by M. Grätzel and the organic thin-film solar cells developed by C. W. Tang are well known (Non-Patent Documents 1 and 2).
Because both have characteristics differing from those of the inorganic solar cells currently in mainstream use, including the fact that they are thin, lightweight films which can be made flexible, and the fact that roll-to-roll production is possible, they are expected to lead to the creation of new markets.
Of these, organic thin-film solar cells (also referred to below as organic photovoltaic cells or “OPVs”) have attracted considerable attention in part because, in addition to having such characteristics as being electrolyte-free and heavy metal compound-free, they have recently been reported by a group at UCLA et al. as having a photoelectric conversion efficiency (abbreviated below as “PCE”) of 10.6% (Non-Patent Document 3).
Also, given that organic thin-film solar cells, compared with existing photoelectric conversion devices that use silicon-based materials, have certain characteristics, such as exhibiting a high photoelectric conversion efficiency even under low illumination, enabling thinner devices and smaller pixels and the ability to provide also the attributes of a color filter, they are noteworthy not only in solar cell applications, but also in image sensor and other photosensor applications (Patent Documents 1 and 2, Non-Patent Document 4). Organic thin-film solar cells, including those for light sensors and other applications, are collectively referred to below as organic photoelectric conversion devices (sometimes abbreviated below as OPVs).
Organic photoelectric conversion devices are constructed of, for example, an active layer (photoelectric conversion layer), charge (hole, electron) collecting layers and electrodes (anode, cathode).
Of these, the active layer and the charge-collecting layers are generally formed by vacuum deposition processes. However, vacuum deposition has drawbacks in terms of, for example, its complexity as a mass production process, the high cost of the equipment, and the efficiency of material utilization.
In light of these drawbacks, water-dispersible polymeric organic conductive materials such as PEDOT/PSS are sometimes used as coating-type materials for forming hole-collecting layers. However, because these are aqueous dispersions, the complete removal of moisture and the control of moisture reabsorption are difficult, which tends to accelerate device deterioration.
Moreover, there remain a variety of challenges facing the use of aqueous dispersions of PEDOT/PSS in mass production. Namely, such dispersions are prone to solids agglomeration, and so defects readily arise in applied films made thereof and the coating equipment has a tendency to clog or corrode, in addition to which the applied films leave something to be desired in terms of heat resistance.